PROJECT SUMMARY In recent years, it has become apparent that a ?microbiota-gut-brain? axis exists where bidirectional communications occur between the gut, its microbiota contents, and the brain. In this proposal, we will develop the hypothesis that this ?microbiota-gut-brain? axis is operational after stroke. That is, stroke produces gut dysbiosis, a pathological change in the gut microbiota, and gut dysbiosis, as occurs with aging, negatively affects outcomes following stroke. Our overall hypothesis is that age-related dysbiosis contributes to the high mortality and poor functional recovery seen after stroke in aged animals. Reversing dysbiosis in aged mice by manipulating the resident bacterial population (?the microbiota?) will lead to enhanced recovery after experimental stroke. We support our hypothesis with strong preliminary data. a) Gut dysbiosis occurred with aging in mice. (b) Changes in the innate and adaptive immunity, occurring with age, were reversed with transfer of young microbiota into aged mice. (c) Gavaging the gut microbiota from young mice (3 months) into aged mice (18-20 months) after experimental stroke improved recovery. (d) A deficit of short chain fatty acids (SCFAs), primarily acetate, propionate, and butyrate, in the gut of aged mice is associated with poor outcome after stroke. (e) Post-stroke gavage of short chain fatty acid producing bacteria (probiotics) enhanced recovery in aged mice. We will develop the idea that age-related changes in T-cell subsets in both the gut and the brain mediate these detrimental effects and that deficiency of SCFAs is responsible for these detrimental T-cell changes. The main goal of this proposal is to understand how the components of the ?microbiota-gut-brain? axis change with age and stroke with a focus on brain inflammation (microglia and brain resident T-cells; Aim 1), gut inflammation (regulatory and gamma delta T-cells; Aim 2) and bacterial products (the short chain fatty acids, butyrate, acetate, and propionate) to enhance the recovery from stroke in aging mice (Aim 3). We will manipulate the biome in entirety (heterochronic fecal transfers), with bacterial metabolites (short chain fatty acids), and with targeted next-generation probiotics. Furthermore, we will determine the role of each short chain fatty acid on stroke recovery by directly infusing into the cecum and colon through a chronically indwelling cannula. Stroke is now the most common cause of long-term disability in the US and the incidence continues to rise with our aging population. The ?microbiota-gut-brain axis? after stroke is a critical and novel area of investigation to fully understand the pathophysiology of stroke and offers a promising approach to therapeutic interventions to improve recovery from stroke, especially in the elderly.